The need for lower cost transceivers is continuously increasing as the use for wireless communication terminals is expanding at a remarkable rate. Among one of the various designs employed in such terminals is an arrangement that includes a super heterodyne receiver, which includes an image-reject filter at the antenna input. Although this arrangement provides for a good quality reception, it tends to be costly and complicated.
Recently, the super heterodyne receiver has been replaced by a less costly design referred to as a low IF receiver which applies. RF image-reject mixing. RF image-reject mixers avoid the need for image-reject filters at the input and enable conversion of radio-frequencies at a greatly reduced cost.
A disadvantage of RF image-reject mixing designs is signal imbalances that are generated by the signal splitter unit that is coupled to the local oscillator employed for demodulation. FIG. 1 illustrates a typical low-IF receiver 10 that employs an image-reject mixing design. Antenna 12 receives radio-frequency signals that are filtered via low-noise amplifier 14, and fed to a mixing demodulator 18 via low-noise amplifier 16. Mixing demodulator 18 includes an RF mixing stage 30, which functions as an intermediate frequency converter of receiver 10. RF mixing stage 30 is configured as a quadrature demodulator comprising an in-phase and quadrature-phase branches respectively. A local oscillator 60 provides a sinusoidal signal to a signal splitter 20. The output ports of signal splitter 20 provide an in-phase frequency signal and a quadrature frequency signal to each of these branches via mixers 22 and 24 respectively, so as to demodulate and shift the frequency range of the received signal from radio-frequency, such as 900 Mhz to an intermediate range such as 100 Khz. Each branch also includes an automatic gain control and filtering unit 26 and an analog to digital converter 27, so as to provide IF digital signals to a second IF mixing stage 28. The IF mixing stage of demodulator 18 functions as a base band demodulator, which is designed to shift the frequency range of signals provided by first mixing stage to a baseband region.
Intermediate frequency (IF) mixing stage 28 includes an in-phase branch that subdivides into two branches 32 and 34. IF mixing stage 28 also includes a quadrature phase branch that subdivides into two branches 36 and 38. Each branch 32 and 34 includes a mixer 40 and 44 respectively, which are configured to mix the in-phase component received from RF mixing stage 30 with an in-phase and quadrature phase local-oscillator signal received from local oscillator 60 so as to provide baseband in-phase signal I1 and baseband quadrature signal IQ. Similarly, each branch 36 and 38 includes a mixer 42 and 48 respectively, which are configured to mix the quadrature phase component received from RF mixing stage 30 with an in-phase and quadrature phase local-oscillator signal received from local oscillator 60 so as to provide baseband in-phase signal Q1 and baseband quadrature signal QQ.
Adders 52 and 54 are configured to add and subtract various baseband components obtained from second mixing stage as will be discussed later in more detail, so as to provide a signal with substantially small image components. It is noted that the image band component signals are caused by interference from adjacent channels which are mixed into the desired signal band intended for receiver 10 due to imbalances in the I/Q paths. The output signal of adders 52 and 54 are then provided to a digital signal processing 56 via digital filters 58 and 60 respectively.
As mentioned above, a significant disadvantage with receiver 10 is the need for extremely accurate splitter unit for the local oscillator to achieve the desired image rejection. Thus, it is important for such receivers that the in-phase and the quadrature phase components of the RF local oscillator 20 are exactly in quadrature and have equal amplitudes. Any phase or amplitude imbalances may directly decrease the image-reject capabilities of the receiver.
A common way to acquire the quadrature signal is by using a RC-CR circuit. When these circuits are employed in an integrated circuit (IC) arrangement, a desired tolerance may not be achieved resulting in a worse than acceptable image rejection. Some designs include poly-phase filters to generate accurate quadrature signals. However, such filters consume relatively high power.
Thus, there is a need for a receiver that provides accurate demodulation with substantially low image band components.